In recent years considerable research has been devoted to the development of magnetostrictive compounds, and in particular rare earth-iron alloys. These developments are summarized by A. E. Clark, Chapter 7, pages 531-589, in "Ferromagnetic Materials", Vol. 1 (Ed. E. P. Wohlfarth, North-Holland Pub. Co., 1980). A major objective of the research has been to develop rare earth-iron alloys with large room temperature magnetostriction constants. Technically important alloys having these properties include alloys of terbium together with dysposium and/or holmium. The relative proportions of the rare earths and the iron are varied to maximize room temperature magnetostriction and minimize magnetic anisotropy. Presently, the most technically advanced alloy of this kind is represented by the formula Tb.sub.x Dy.sub.1-x Fe.sub.1.5-2.0 wherein x is a number from 0.27 to 0.35. An optimized ratio is Tb.sub.0.3 Dy.sub.0.7 Fe.sub.1.9 which is known as terfenol-D, as described in U.S. Pat. No. 4,308,474.
Such rare earth-iron alloys are true compounds and can exist in crystalline or polycrystalline form. In preparing elongated bodies (viz. rods) from such alloys, grain-orientation of the crystals is essential for achieving high magnetostriction. An axial grain orientation of the crystallites not only increases the magnetostriction constant but also reduces internal losses at the grain boundaries. This is particularly important in applications where a high magnetostriction at low applied fields is required. (See Clark, cited above, pages 545-547).
As cast magnetostrictive rods, such as those formed from the Terfenol-type alloy, can be processed by a "float zone" method, as described in McMasters U.S. Pat. No. 4,609,402. The as-cast rod is subjected to progressive zone melting and resolidification. In optimized embodiments of this method, the resulting rod is essentially a single elongated crystal extending for the length of the rod. The longitudinally-aligned single crystals have a &lt;112&gt; direction parallel to the growth axis, and twin plane boundaries running perpendicular to the &lt;111&gt; direction of the crystal. [See Verhoeven, et al. (1987), Met. Trans., 118A: 223-231.] As described by Verhoeven, et al. magnetostrictive rods with generally similar properties can be produced by a directional freezing technique in which a liquified column of the alloy is progressively solidified. That procedure results in aligned polycrystals with a &lt;112&gt; direction nearly parallel to the growth axis. Single crystal rods produced by the float zone method, and polycrystalline rods produced by the zone solidification method exhibit comparable magnetostrictive responses as originally produced, both when uncompressed and when subjected to axial compression.
Because of the much greater response of compressed rods, it has been the practice to employ magnetostrictive rods under a high degree of compression. However, it has been desired to produce rods which can provide an adequate response at less applied pressure, or optimally at zero pressure. Such rods would facilitate the use of lower strength magnetic fields, and less current would be needed.
McMasters U.S. Pat. No. 4,609,402 states that the as-cast rods may be subjected to heat treatment to obtain phase equilibrium (col. 7, lines 16-20). This disclosure reads: "The directionally oriented rods may be heat treated in a vacuum tube furnace chamber to achieve phase equilibria. This can be accomplished at 950.degree. C. for a period of five days. A slow cool down period, usually overnight, is desirable in order to avoid cracking." Such heat treatment tends to improve magnetostrictive response of compressed rods, but the uncompressed response remains of the same order. Heretofore, no method has been known for producing magnetostrictive rods which exhibit a magnetostrictive strain when uncompressed of comparable magnitude to that obtained under compression.